Tell arch to turn 180 degrees, then kick him in butt....!
Knee wall spaces are about the worst to put a unit in without sheetrock, they're really bad to replace a unit in.
Don't put any structural stuff directly in front of the unit! You'll just have to cut it out later. Since you're looking to save floor space you won't be climbing in a closet crawling in front of unit, sounds like doors will be attached to wall to take off and gain access directly.
So you're looking at measuring up from floor the installed height(includes drain pan and supports) to the rafter plus a few inches to play with(put mark on floor). Measure horizontally out the depth of furnace or drain pan (the deeper one) plus a few inches. This will be the back side of wall.
Remember- access to repair, access to replace, without being a contorsionist or tearing the house down.
Did I mention the shoe leather enema?
Col 3:23
questions asked, answers received, ignorance abated